Silicon compounds silyl enolates

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

Addition of silyl enol ethers to nitroarenes2 In the presence of 1 equiv. of TASF, silyl enol ethers add to nitroalkenes to form unstable ortho and/or para nitronates, which are oxidized in situ by Br2 or DDQ to nitroaryl carbonyl compounds. The position of substitution depends on the substitution pattern of the arene and the size of the silicon reagent. With less hindered silyl derivatives ortho addition is strongly favored. 

One silicon tethered example that is unique in its selectivity is the cinnamyl tethered silyl enol ether shown in Sch. 17. Unlike all of the other silyl tethered examples, this compound gives a photoadduct that is the result of a cross 2+2. However, it is the product expected if the cycloaddition is a stepwise process involving radical intermediates. It is also the product expected if the reaction pathway is controlled by 7i-stacking. 

In contrast, the related silyl enol ethers are available by mild selective transformations from carbonyl compounds or other precursors 55). Their stability and that of products derived from these alkenes can easily be regulated by choosing suitable substituents at silicon. Selective cleavage of a Si—O-bond is possible with fluoride reagents under very mild conditions, and this is why cyclopropane ring opening can now be performed with high chemoselectivity. 

Silyl enol ethers and ketene silyl acetals react with tetraphenylbismuth fluoride under neutral conditions to give the corresponding a-monophenylated carbonyl compounds in good yields together with triphenylbismuthane and fluorotrimethylsilane (Equation (111)).181 The reaction is likely to be initiated by the nucleophilic attack of the fluoride ion on the silicon atom, and the regiochemistry of arylation is governed by the structure of silyl enolates. 

If the 1,5-diearbonyl compound is required, then an aqueous work-up with either acid or base cleaves the silicon-oxygen bond in the product but the value of silyl enol ethers is that they can undergo synthetically useful reactions other than just hydrolysis. Addition of the silyl enol ether derived from aeetophenone (PhCOMe) to a disubstituted enone promoted by titanium tetrachloride is very rapid and gives the diketone product in good yield even though a quaternary carbon atom is created in the conjugate addition, This is a typical example of this very powerful class of conjugate addition reactions. 

The other side of the coin is that the S 2 reaction at carbon is not much affected by partial positive [ charge (5+) on the carbon a tom. The Sn2 reaction at silicon is affected by the charge on silicon. The r most electrophilic silicon compounds are the silyl triflates and it is estimated that they react some 108-109 times faster with oxygen nucleophiles than do silyl chlorides. Trimethylsilyl triflate is, in fact, an excellent Lewis acid and can be used to form acetals or silyl enol ethers from carbonyl compounds, and to react these two together in aldol-style reactions. In all three reactions the triflate attacks an oxygen atom. 

Even if silicon chemistry is new to you, you should by now have a picture of stable compounds with C-Si bonds and selective reaction with fluoride. You are already familiar with silyl enol ethers as nucleophilic enolate equivalents and allyl silanes resemble these in many ways. The missing link is the i-silyl effect. A Si atom stabilises a cation in the p-posit ion by overlap of the populated and relatively high energy C-Si c-orbital with the empty p orbital of the cation. This overlap is already present in the preferred conformation 95a of the allyl silane 95 as an anti-bonding interaction 95b between the C-Si c-orbital and the n orbital of the double bond. The resultant molecular orbital (the new HOMO) 95c increases the nucleophilic reactivity of the carbon atom in the y-position. 

Although the mechanism of the Mukaiyama reaction is not yet fully understood, several points have now been firmly established (a) a Lewis acid enolate is not involved (b) the Lewis acid activates the carbonyl group for the nucleophilic addition and (c) the Si—O bond is cleaved by nucleophilic attack of the anionic species, generally halide, on silicon. Point (a) has been established by the use of INEPT- Si NMR spectroscopy. Moreover, trichlorotitanium enolates have been synthesized, characterized and shown to give a completely different stereochemical outcome than the TiCU-mediated reactions of silyl enol ethers. Complexes between Lewis acids and carbonyl compounds have been isolated and characterized by X-ray crystallography and recently by NMR spectrometry. On the basis of these observations closed transition structures will not be considered here open transition structures with no intimate involvement between the silyl enol ether and the Lewis acid offer the best rationale for the after the fact interpretation of the stereochemical results and the best model for stereochemical predictions. 

Reaction with ei,fi-Unsaturated Sulfoxides. The reaction of TMSI with a, -unsaturated sulfoxides in chloroform at ambient temperature is a mild, efficient, and general method for the preparation of carbonyl compounds (eq 63). The proposed reaction mechanism is shown in eq 63. Formation of a strong oxygen-silicon bond is followed by reduction of the sulfur function and oxidation of iodide to iodine, the latter precipitating in chloroform. The trimethylsiloxy anion attacks the unsaturated carbon linked to the sulfur function, which leaves the substrate, allowing the formation of the sUyl enol ether species. Finally, hydrolysis converts the silyl enol ether into the carbonyl compound. ... 

Scheme 8.95. The formation of a generic silyl enol ether (R = alkyl, alkaryl, aryl, etc.) from a generic carbonyl compound (aldehyde or ketone) with imidazole as base in methylene chloride solvent. It is argued that the silylation occurs preferentially on oxygen because of the strength of the silicon-oxygen bond. The silyl enol ether can be reconverted to the corresponding aldehyde or ketone by treatment with tetra- -butylammonimn fluoride ( -Bu4N" E).

The Lewis acid mediated aldol-type reaction of nucleophilic alkenes (alkyl enol ethers, enolesters) has been investigated since 1939 and is well documented in the literature. The first breakthrough in this field was in 1973-74 when Mukaiyama and coworkers found that silyl enol ethers are much more effective than alkyl enol ethers in the Lewis acid mediated additions to carbonyl compounds. This is related to the fact that silicon is markedly more electropositive than carbon (1.64 vs. 2.35), resulting in a stronger polarization of Si-0 bonds and in a stronger tendency for nucleophilic attacks at silicon... 

Conjugate Addition. TBDMS triflate has been used to promote the conjugate addition of carbon- and heteroatom-based nucleophiles to a range of a./S-unsaturated carbonyl compounds, in both stoichiometric and catalytic quantities. In some cases, the silyl enol ether is isolated, in other cases, it is implied as an intermediate but hydrolyzed either in situ or by addition of an acid or a reagent known to cleave a carbon-silicon bond e.g., TBAF. Examples of carbon-based nucleophiles are shown in eqs 22-26. 

Vinyl siloxonium ion 14 also serves as an important intermediate en route to conjugate adducts of unsaturated carbonyl compounds. For example, treatment of cyclohexenone (13) with reactive silyl electrophiles affords y-functionalized silyl enol ethers 15 and 16 suitable for subsequent synthetic transformations (eq 4). The temporary silicon tether (TST) strategy has been updated (2010) by an excellent review focusing upon metal-mediated reactions. The inception of this strategy is attributable to Nishiyama and Itoh who reported the radical cycUzation of acyclic bromomethyl silyl ethers to sUoxanes and their subsequent oxidation to 1,3-diols. Shortly thereafter, the group of Gilbert... 

---